I'm confused - long+low, or HITT??

I am a 68 male trying to learn to ride longer (currently do 25-30) - traditional advice was build a base by lots of low intensity riding (long and low heart rate). Now all the advice is better to do high intensity work. I would like to work up to a 60 miler- so back to hours on the stationary bike at HR under 115?? or hit the spin class for an hour around 140-150 HR? suggestions?

Also researching what to eat/drink on longer (and hotter) rides. Don't think the oreos are going to cut it

Len