I used a Garmin Edge with a heart rate monitor last season, mainly for my road riding. I have a route I use for personal time trials, and started by setting a "base performance level". The first couple of trips out on my time trial loop, I rode as hard as my body would allow. I began the season with a heart rate average that ran in the 140-145 range, and peaked in the 175-180 range during hill climbs and sprints. My average speed for the loop, which is in the rolling hills of southern Indiana, was in the 15.5 mph range.
Over the summer and fall, along with other riding, I would ride the same loop occasionally, riding hard to keeping my heart rate up, and would try to hit the hills and sprints as hard or harder than I did the time before. With the Garmin, I had the advantage of knowing things like how fast I'd climbed a hill on a previous ride and would try to climb it faster. I had a few rides early in season where my heart rate for the route was a little higher, but by the end of the season, I was running the same loop at 17.5-17.9 mph with the same heart rate averages I'd started the year with.