I remember a few years back when 5 miles was quite an accomplishment. I'm 64 now and have a similar bike - a Specialized Crosstrail Sport. I can easily ride 20 miles now and have over 200 miles logged on Strava this month. I don't pay much attention to speed - I find that whatever day I'm riding I find a pace that is comfortable for me that day and keep at it. I feel better after a ride for sure - and at some point during most rides I get a feeling of happiness (for lack of a better word) that comes across me. It feels strange now to log less than 10 miles! I slowly built up my mileage to get to where I am so my advise is to just go slow and bump up the distance in small increments when you feel like you're ready.