Is it normal to stop so much because of thighs burning?
More than likely it is normal and frankly your legs probably aren't used to it. 17 miles is a long way for a beginner so I would pat yourself on the back for that.

You may also benefit from a proper fit from a bike shop. As a lot of beginners either have their seat too high or too low which can limit your power and actually cause you to tire quicker, although a fit takes a lot more into consideration that just that. Either way it would probably be worthwhile to at least look up a good guide and check your posture, it can make a big difference.

Regarding your age, I am 32, and know several people in their 60's that could leave me behind going up hill (or anywhere) with ease. 

I am in okay shape, and have been riding off and on for several years and still have to stop from time to time. It does get better though.

You may also take a look at your diet and pre-ride foods, it is also important that if you are going on a long ride you take some snacks for along the way so that your body has some fuel to burn. Eating 30 minutes to 1 hour before a ride vs not eating at all can make a monumental difference. 

Bottom line you have made a healthy choice and a fun one to improve your own health and way of life! Keep at it and don't give up or feel bad if you need to stop and catch your breath from time to time, the most important thing is you are out riding!