I feel your pain, bro. I was diagnosed with Type 2 Diabetes and had no choice but to start getting some exercise. I started with an old Univega Alpina steel framed MTB and set it up for the road. I mainly ride trails and paths like you stated. I went from 340 to a current 284 and the Uni is still going strong.

I inadvertently became the "new" owner of a 2003 Lemond Nevada City bare frame that is slowly being built for centuries next year. I think there is a thread in the touring forum about converted MTB's I'll see if I can find it.

Found it:
http://www.bikeforums.net/showthread...MTB-conversion

A little searching can find a suitable donor in your price range and in a year or so of socking away some funds, you can go get what you want with less compromising in budget.

Good luck.