Cadence and heart rate are the two key measures that I use. Over a 30 to 40 mile solo ride, I can exert a strong and steady effort. Due to terrain and wind, my speed will vary greatly between 14 and 24 mph. So, I do not dwell on my speed from second to second. In the end, my average speed is all that matters, after the ride. On average, it is a simple measure of my average fitness. On the ride, I want to know, or work on, my cadence, my power delivery. Since I am reasonably fit, my legs do not tire easily. I can pedal at a cadence and for a period of time that will drive my heart rate to any targeted training zone. Knowing how fast I can spin and how hard I can drive my heart is worthwhile data for me.
November, Trek OCLV, Bianchi Castro Valley commuter
#1 is yourself - if you really feel like cr*p, you probably need a day off the bike (or at least take it easy that ride). If you feel GREAT!, go out and hammer. #2 - with the cyclecomputer you have now, you can track long term changes in performance over the identical route - I've used a 4-5 mile section of a local bike trail and a couple of local hills for years, and started tracking my commute when it went from 4 miles to 11 each way. When the 5 minute climb becomes a 4 minutes climb, or you cover the section that took 20 minutes & a tailwind now in 15, you KNOW you're getting better. Centuries (and other distance rides) are another good year-to-year measure. Group rides and races are the ultimate arbiters if you want to compare yourself to others vs your personal bests. In fact, riding with another cyclist a bit stronger than myself is the best motivation to push myself - it's mentally hard to push yourself solo, especially longer efforts.
HRMs and power meters are great for pacing yourself during an activity, along with RPE. HRMs are cheaper and good enough for many people. I used one for several years. Power is the current ultimate, but you pay for it ("cheapest" reliable way is a wired or wireless powertap wheel and a unit that can receive and record ANT+ data; there is an iPhone dongle and several bike computers, Garmin watches/computers, and other devices that do it). $500-$600 or more for the wheel, maybe less if you find a racer who was meticulous about his/her gear who's getting new wheels or switching to a different power system (what I did). Adding a cadence sensor may also be useful.