Cross country skiing
Cross country skiing is a very efficient way of travelling great distances. The skis are narrower than the usual downhill and slalom skis, and the poles are longer to help you pole along. You can go cross country in a prepared track or outside in fresh snow. Skis to go outside of the tracks tend to be a bit wider than the skis which you use in tracks, this is to help you to float better in the snow outside of the tracks. Boots are usually made of leather or some synthetic flexible fabric.
You might want to go for a skiing trip instead of jogging, for variation at a ski resort or as a means to get to the true wilderness in wintertime.
Skiing in prepared tracks and going true cross country are different worlds. The same equipment may work quite well in suitable conditions, but extreme variants for either are miserable for the other. There is also a huge difference between taking a tour in the neighbourhood and travelling with equipment for staying overnight.
There are two different techniques for skiing cross country: "classic" and "skate skiing", where the former means mostly gliding forward with the skis in the tracks, while the latter resembles skating. In deep fresh snow classic is the only way, likewise if there is no space besides the tracks on a prepared route. Nowadays prepared routes often offer space for both. If the temperature has varied around freezing point (0°C) there may be hard snow on or near the surface allowing the skating technique on fields (lakes and rivers with thin snow cover also invite to this).
The mid part of the ski should touch the track firmly when kicking, but rise when gliding, which means the skis have to be chosen to fit your weight. Some skis are more forgiving, which is an advantage when having a backpack that might be more or less heavy. There are also differences in how carefully the ski has to be prepared (waxed) for the specific kind of snow facing you on a route. Long and wide skis are good in thick loose snow in open terrain, but long skis are cumbersome in woods, bushes and steep hills and extremely wide skis may not fit in prepared tracks (also some bindings may be problematic).
When getting the equipment, be prepared to choose between several options depending on what you are going to do and what you are prepared to pay (at least in specialist shops, other may offer only one kind). The skis, bindings, boots and poles may be optimized for either classic or skate skiing, for tracks or for true cross-country with deep loose snow, for mostly flat terrain or offering a possibility also to downhill skiing with the telemark technique. Skis, bindings and boots should match and are often sold as a package; consider whether you want to be able to replace some without replacing all of them.
Learn to ski before you try longer trips on your own. There are courses at many ski resorts, but if you are able to train at home you can benefit from more advanced advice or more easily head for the real adventure at your destination.
You might want to get skis, poles, boots and other equipment at home or buy or rent at or near the destination. In the latter case, find out what is available, when and where.
Learn about your destination: what kind of routes are available, what services at the hub and en route, what to expect about the weather etcetera.
- Ski resorts. There are cross country routes around many of them.
- National parks and similar at the right latitudes or altitudes.
- The back yard or near-by park at many destinations in the north.
Long distance skiing 
Think of the skiing route as of a similar hiking route. You might have a toboggan, ahkio, pulk or similar to pull some of your equipment instead of carrying, which means you can have more stuff if you really need to (such as when crossing Greenland). You will need warmer (heavier) spare clothes, more food (quite a lot of energy is used to staying warm), a better (heavier) sleeping bag, and a tent even if you plan using cottages (a snow storm might force you to camp earlier). In good conditions you will be able to cover greater distances.
There might be prepared tracks most of the way, but be prepared to cope without them, as you might have to diverge from the route or you might loose them in snowy weather.
In many types of terrain heavy snowfall will reduce your ability to orienter - it is difficult to keep a course while seeing only snow. In some cases wind or quite moderate snowfall will be enough. If above the tree line, you must be prepared to find a safe place and wait as long as needed.
When in a national park or similar there are often cottages for lodging. They may be unlocked and unmanned, available only after reservation or full-fledged hotels. Be sure to find out the local conventions and the locations of cottages you might want to use.
In some locations you can hire a guide or buy a full service package including somebody making tracks with a snowmobile, preparing the food and arranging visits and programme on the way. In others (especially if you are on a budget) you will be more or less on your own, which means you need experienced company and some training yourself.
Stay safe 
If there is snow for skiing, the weather is or may turn cold enough to pose a risk. See travelling in cold weather for general advice. Always tell somebody where you are going (including alternative routes), when to expect you back - and when to call for help if you did not turn up.
You do not want to go for a longer trip in heavy clothing, get wet by sweat, get tired, slow down and get cold. Instead you want to be able to use quite light clothing most of the time (covering also head and hands, though), but also to protect yourself against wind and have a sweater ready for the breaks.
Sunny weather and snow means very much light. Sunglasses might be necessary on longer trips to avoid snow blindness. Also sunburn is possible, which might be hard to notice in time in otherwise cold weather.
When going real cross-country or on longer trips without an experienced guide you need to be able to orienter. Do have a suitable map, a compass and the needed skills. Also be prepared for changes in weather. Will you get to your base before dark if you loose the advantage of prepared tracks because of snowfall?
If you intend to cross rivers or lakes, or use them as part of your route, be sure to know how to avoid thin ice, how to get up if you are unlucky anyway and how to keep yourself warm afterwards.
In mountainous areas there might be cracks or steep hill sides obscured by snow. Always know where you are when near potentially dangerous places.
There may be risk of avalanches or snow storms on some popular cross country routes. Do know how to cope.
Do not leave your party or your equipment when snowfall may hinder you from finding them.