Smiles per mile - or in your case, Smiles per kilometer. That's my training guide.
Welcome and enjoy. I started bicycling at age 58, so you will have to pedal fast to catch up!
Truly, don't punish yourself, unles you are in the masochism sub forum of the 50+ forum (and we have a few of those, I believe). Keep it fun and you will bicycle forever. (Well, almost).