90-100 BPM is pretty high, but some people just have high HR's. I have a friend who has allergies who is always recovering from some bodily ailment and his HR is always really high. I think his typical riding HR is in the 180's and he has a max in the 200's. I find that insane because if I hit 180-ish I'm dying, but that's because HR is different for everyone.
Finding LTHR is basically finding what you can hold for an hour maxed out. Do a good warm-up and then do a 20 minute TT effort. Multiply by your average HR for the 20 minute by 0.95 and you have you're LTHR. Or if you can stomach it, do a 1-hour TT effort and see what your average HR is for the 1-hour.
Threshold zone is your 1-hour of power zone, if you're only riding for and hour, that's your max zone. If you're riging less, you'll have a slightly higher HR in that zone or go up one zone. If you are going longer, better to stay below the threshold zone into endurance (long distance riding, 3 hours or more max pacing) or tempo (hard endurance at just sub-threshold, 2-3 hour hard effort rides).
If you're mountain bike it gets harder to use HR as a constant gauge because there's many micro efforts that are never logged by an HR monitor since they start and end before your HR changes. That's when power meters come in handy, but that's a whole different scenario.