Here's some simple rules of thumb that work for me, I'm in good shape but weigh 220 lbs so I'm not a fast climber and obviously all kinds of YMMV. These apply to typical fast fun rides, not training rides.

Rule of Thumb A - equal length rides between 35 and 75 miles (loops, down = up), my average moving speed on relatively flat ride ends up about 10-15% slower on a moderately hilly ride, 15-30% slower on a crazy hilly ride.

Despite my gravitational advantage going down hills, I'm not a fast descender - here in Maryland the roads I do have too many deer, squirrels, potholes, dogs, cars, etc. and I am too wimpy to make the most of steep downhills. Doing Going to the Sun Road in Glacier National Park last year I had some screamer descents - not much of that around here.

Rules of Thumb 2. Staring at the bike computer on long climbs, I estimate that for every 2% the incline goes up, my average speed goes down about 20%. This holds true up to about 6%, starts to get more drastic drop after that. The last 10 miles or so up to Logan Pass on Going to the Sun road is a constant 6% and the rule held there - we were on rental hybrids, no bike computer, just going by time.