Cycle Fitness
how fast should i be able to cycle 8 miles?
I’m 14, and doing some cycling to build up fitness and strength as sidetraining for cross country skiing 
I can currently do 7.6miles ( according to google maps ) in just over 30 minutes and was wondering if thats any good?
A mile every 4 minutes (15 miles/ hour averaging) is a good pace for a beginner. So 8 miles in 32 minutes would be a nice early spring goal for you.
Even with hills this should be doable as long as you are ending in the same spot as your started. I have a 30 mile loop that I like to do in 90 min that includes a 1000ft climb and then decent, you just have to make up the time on the way down.