I'm doing this ride on Saturday which has 10,000' of (accumulated) climbing over 100 miles . . . though the maximum elevation is only around 2,500', maybe even less than that and lowest elevation is around 100'. Extended sections of 10%+, some as much as 18%. We'll be going up and down a lot . . . though I haven't done it, I know it is going to be "tough".

The next closest thing I've actually done was 10,000' in 125 miles, and I considered that pretty challenging, as did quite a few other riders I talked to. I'd also personally consider it "mountainous", though it only seems to qualify as "hilly" by johnny99's club (OT: which club?).

I've done a couple of shorter rides which have exceeded 5,000' of climbing in under 30 miles (again, up-down-up-down type of deals, not 5,000' elevation), which qualify as "mountainous" . . . though I did not consider these "tough", perhaps because of the shorter distance.

I have not yet ridden above 4,000' of elevation, though I've done quite a bit of mountaineering up to about 14,000' or so. In that activity, I don't start to notice the effects of elevation (reduced oxygen) until around 7 or 8,000'. I'd expect to notice it sooner cycling