Exercising outdoors might be great for your psyche, but it's not so good for your skin. Even if you're only out for a 30-minute bike ride, you need to wear sunscreen. Your daily moisturizer with SPF just isn't going to cut it though, since the heat of the sun and all your hard work will make you sweat. The lotion may end up running into your eyes, plus all that sweating will also make your sunscreen less effective. So pick up some sweat-proof sunscreen.
Whichever sunscreen you choose, make sure it says that it's sweat proof or sweat-resistant on the label. If you worry about breaking out, look for one that also says "non-comedogenic." Apply sunscreen 30 minutes before you head out, and if you're planning on being out for longer than two hours, bring some sunscreen along so you can reapply as needed.