A heart rate monitor is Very useful for following a training plan, and is nice, but not necessary, to assess fitness and progress.
As you improve your fitness your resting prone heart rate will go down, your times will go down, and your recovery rate will improve.
You need a "challenge", a watch, and a journal.
My "challenge" is a 3 mile hill that climbs 1200'. I walk up as fast/efficiently as I can. I track my time to the top. I track how long it takes my heart rate to drop below 100 after I stop. I track my morning heart rate, prone, before I get out of bed (one eye open, watching the sweep hand).
In the winters when there is to much ice I use a indoor rower. In the summers I'm also on a bike.
A side benefit to this approach is repeatability day-by-day, as well as year-by-year. Resting heart rate and recovery time will tell you everything you need to know.
Edit: Also, on a 10 day hike, when you wake up, check your heart rate. If it's "typical" be happy, if it high, back off a bit. When you get to the top of a pass, check your recovery. It's interesting to watch your "bounce back" as a trip progresses. No "monitor" required.