Make sure you are using your gears (assuming you have gears). Don't try pushing big gears, tires out your muscles before your cardio is done in. Aim for 60-80 rpm cadence, maybe 40-60 up hills. Your cadence should be smoother and probably faster as you get used to riding.
Your rear may hurt on following days but keep riding and that will (should) go away. It will feel tender like a bruise maybe with hotspots. After 5 or 10 minutes of the initial hurt you won't feel it as bad.
Keep at it, the improvements will follow. 36 is still young
