Two week taper. I’m not sure the elevation, but I ran 12 miles in the same area last year and the elevation gain was 1500 ft for that run according to Strava.
As for back to back runs, all I can say for now is “we’ll see.” I’d like to get on that level. It also depends on how you define a long run. Could I run another 35k today? Absolutely not lol.
Last time this happened to me it was because I didn’t eat enough beforehand.
Usually I eat a decent lunch then go for my run a couple hours later - maybe eating a banana 15-30 min before. That day because there were thunderstorms forecasted I skipped lunch and just had a banana and a cliff bar before my run. Got tired pretty early on that day.