Is this a good backpack?

If all you're doing is day hikes and bike rides, then an 85L backpack is probably more than what you need. A 50L pack is probably going to be plenty of space for the usual water, sandwiches and spare clothing you might carry. I would however go to a camping or hiking shop and have a look at an 85L and 50L backpack to compare them and make a decision from there.