Hi and Welcome.
For me, doing laps of anything (unless its a track and i'm racing) seems a really hard way to put miles under my wheels. I would suggest if you want to get your miles up towards a 100 miler then do the opposite. ride some place and dont ride back...get the train home instead.
The thrill in not knowing what's coming keeps the ride fresh and more enjoyable. When I was training for a big tour last year, I used to head off to the coast and if I got fed up would bail and train it home. Sometimes I didn't get bored and ended up riding all day and doing 100 miles without even planning to.
also, try to share the big rides with friends (I appreciate that looking for ride buddies is one of the reasons you are positing), add plenty of stops for cake and coffee and the whole thing takes on a different perspective.
Those sportives that you mention, or perhaps even group rides to the coast (check out the FNRTTC) are also a good way to familiarise yourself with higher mileage.
Good luck