You won't be able to hold 90% of MHR, but with training 85% should be possible. You want to lay down some serious base. Lay out a good periodized training program. You want a light, stiff bike. I have a Mt. Washington cast-off, a CAAD9 that only had one ring and three cogs in the back when it was in climbing trim. People even cut down their bars. Mostly, get your endurance up and lose weight.