Hello Tom . . . I use my MTB all year round.

Some personal & general facts:

- I used to walk the same distance to work (40 minutes) and I used to get back home exhausted/beat, also didn't lose any weight. But now using the bike, the same distance takes me only 15 minutes, I get home not too tired . . . and I've LOST weight too! So cycling burns calories - I'm average weight at 73 Kg.

- After a full year, now I cycle a minimum of 20 minutes each day at a steady rate of 60 rev per minutes (maintaining a moderate exertion level, ie about up to 75% of my max heart rate).

- Fitness definition - In general, people need a MINIMUM of 20-30 minutes of 'moderate' exercise about 3 times a week.

I'm sure most of you know the facts . . . If not then I urge you to read the guide from the USA National Institute on Aging (pdf version also available)
http://www.niapublications.org/exercisebook/exercisebook.asp