Just make sure you are nice and hydrated before your ride and you'll be fine.
Where's your ride? It's been ages since my last long ride =(
I'm cycling from Selby near York to Masham near the Yorkshire dale, at 4500ft of ascent it's pretty easy and quite a mixed rather than hilly 120 mile ride. I did 116 miles on Tuesday.
Some rides we usually go up to Tan Hill, they're between 140 and 160 miles.
I like Wales for hills, Manchester Wheelers do some rides there and it's pretty hilly, there is this 33% hill near abergele or w/e it's called and that's a fun climb