There's no periodization, ie., no weeks where you recover and supercompensate.
ericm is absolutely right. You only have eight weeks. You need to ramp up your distance a lot sooner. My suggestion, if you can afford the flexibility, is
1) break up your week into 11 periods of five days each.
2) Start your long ride at 25 miles
3) Do one hard workout each period.
4) Increase your volume two periods in a row and drop down to rest
P1 - 25 mile ride, 40 minutes easy, 40 minute tempo.
P2 - 30 mile ride, 40 minutes easy, 40 minutes hills
P3 - 20 mile ride, 60 minutes easy, 20 minutes strength
P4 - 35 mile ride, 50 minutes easy, 40 LT (break this up as you see fit. 4x10 or whatever)
P5 - 40 mile ride, 50 minutes easy, 20 minutes hills @Vo2max
P6 - 30 mile ride, 40 minutes easy, 30 minutes tempo
P7 - 45 mile ride, 60 minutes easy, 40 LT
P8 - 50 mile ride, 20 minutes VO2max, 40 minutes spin easy, 40 minutes tempo (4 workouts)
P9 - 40 mile ride, 20 minutes VO2max, 40 minutes easy w/ 5s speed bursts every 2 minutes
P10 - 35 mile ride, 20 minutes VO2max, 40 minutes LT or Hills
P11- race week. 15 mile ride, 40 minutes easy with 5s speed bursts every 5 minutes
You get 50 days w/ 30 workouts not counting P11. Two days of rest per period. The final ten days are all about tapering. Don't worry about your HR. Just finish the distance rides. You need to do the distance, not go by time. XC skiers go by time. Runners and cyclists and swimmers go by distance. You need to do the distance. No cheating. Start real slow and make sure you finish. Don't worry about your LTHR either. By the time you get to P4 you will have a rough idea of what it might be. By the time you get to P7 or P8 you should have that pretty dialed in.
PS: 75% of MHR for 6h is not standard. That's general aerobic range. Most distance athletes in any discipline do their distance workouts slightly above brisk walk HR. (Ivan Babikov however is the exception)
PPS: Also the plan takes you only to 50 miles. Tough it out the last 15.
Practice your nutrition and hydration on every ride.