rsbob 

I know that power meters are ‘the thing’ but they are not inexpensive either.

I would suggest a heart rate monitor paired to a cycle computer which can be easily monitored. Get Strava which will read your monitor and categorize your efforts into various zones base on effort. You will quickly learn what level of effort (BPM) you can sustain for a prolonged period and at higher heart rates. You will also know how long you can sustain near maximum effort and then ride your hill base on running near maximum and then kill it at the end.

I did a 60,000’ climbing challenge in November and 3/4s of the way through it my speed and knowledge of how to pace myself greatly improved. as my coach would say, “To win in the mountains, you must train in the mountains”. Unfortunately he was right.