Originally Posted by
TimothyH
Or a heart rate monitor
+1 Also you can observe your breathing. Can you talk as you ride? (Classic recovery ride.) Is your breathing even and deep? Starting to get ragged? Heard a block away?
The measuring tools (power, HR, etc.) will not give you the whole picture until you've ridden and used them enough to know what works for your body. Ie, what is your "max" or "baseline". Whatever you buy (if you do), observe your breath and general feelings as you use them. You will start to learn what works for your body and what doesn't. As your fitness improves, your speeds at a given level of breathing or heart rate will increase. You will also get more and more in tune with your body and start to know where it is without tools.