As I reach my 35th birthday, I have become more and more health conscious. I'm 5'10", 158 pounds. I try to cycle at least once a week in the city, whether it's at the park, next to the beach, etc. for at least 10-15 miles. My style is more on the recreational side because I want to actually enjoy cycling. However, there have been a few times where I've almost reached the 30 mile mark.
Unfortunately, that seems to be my breaking point. The first time it happened, I felt "exhaustion" of my muscles in both my legs' upper thighs. I was dismounting my bike at this point, but a horrible cramp followed after, dropping me to the ground. To add insult to injury, my muscle spasm wasn't just localized to my thighs...let's just say that I'm glad my cycling shorts were black!
After I recovered, I braced myself onto my bicycle as I slowly walked back to my car. It was a long and wet walk, but I got home okay and rested up.
It was several months after until I decided to do another 30 mile attempt. The same thing happened--my muscles seem to "curl up" at around the 28 mile mark. Again, I was hit with a cramp, but at least my shorts stayed dry this time
Anyway, my question is how to increase my muscles' endurance safely. I don't ride particularly hard. In fact, I take lots of breaks for Instagram shots, but there are a lot of hills in San Francisco, which makes me go into overdrive when I pedal uphill.
Any advice is greatly appreciated. Thanks!