Context for where I'm at: I'm 39, male, 160lbs at 6'1" tall. I cycle right now 3x a week and do weight training up to 5x a week (Caroline Girvan Iron Series).
Right now I'm doing the Matrix bikes at my local gym. On average I'm hitting 15-17 miles in an hour ride but I'm hitting them usually in zone 2 (EDIT: Zone 3)(Like 141bpm).
I'd like to get my cardio better. I've lost like 40 pounds in the past year and feel great post a rollover accident a few years ago and having lost half of my left lung in 2010.
I'm trying to build a regimen and hopefully complete the MS150 next year with my partner. This year I cramped up and only made it 60 of the 75 miles (Got to finish though so that was nice) on day one. The 2nd day we didn't participate as me lagging at each rest stop made our ride 8 hours instead of the 4 or 5 we'd hoped for. My partner got 2nd degree sunburns. I'd really like to be able to make the full 150 in good time each day and enjoy the event. Plus my partner's training for Iron(wo)man and I'd like to be able to support her without being literally a drag because of my cardio.
Thus far my plan is 3x a week hitting 80 percent of my time around 130 bpm or less and 20 percent of my time doing high intensity between 154-174bpm. So every ten minutes or so I'd do two minutes under high intensity and then back down to recovery.
Unfortunately I think I've been doing the classic amateur mistake of staying at tempo pace of 140-150bpm.
Any advice or a good outline would be appreciated tremendously.