Good job with the weight loss! I commute 5 days a week, 10 1/2 miles round trip. I've been commuting to the same job for about 13 years, so for me to build fitness or challenge myself, I take longer rides on my off days that are between 30 and 75 miles depending on the weather, how I'm feeling, etc... I find that changing your route up every now and again or adding a mile or two will help you slowly get used to riding longer and stronger. Also, throw some climbing in as you see fit. My return from work involves about 2 1/2 miles of 5-8% climbing, so I now feel more confident on hills and my pacing and cadence during climbs is stronger. In the end, do what feels right and keeps you serene. Keep up the great work!