I've been downhill skiing for more than 50 years, and despite the toll of age I still enjoy the rush that a fast run gives me. However, there are not a lot of 80-plus seniors on the downhill ski runs around us.But there's another kind of skiing that seniors who walk comfortably can enjoy, and that's cross country skiing. It's a different world from the fancy downhill stuff. It's safer, costs less and offers outdoor activity when many hiking trails are snowbound. And it brings one in contact with Nature in ways otherwise missed.As always, it's wise to check with …