I know that feeling well! Went when my children were tiny ( first one was 6 weeks old) with exactly that in mind!
There are now 3 Olympic sized pools, you can swim on your own or they have some lessons to help improve your stroke ( or racing starts if that's useful). You can also swim in a recreational pool ( can be bracing as not heated) or have a guided swim session in the lagoon on site if you fancy open water ( a lot of people training for triathlons ). You can also run on the track and there are track sessions run by the staff, or go on organised off road runs , which is a nice way of meeting people. There are morning walks and runs 3 or 5 k, which is running or walking with a group.
Pilates sessions are good, basic and improvers mat work. Can also book into reformer classes but these are extremely popular and quite tricky to get onto. Yoga was good too, and Pilates mat and yoga outside. There's are classes like bump pump, body attack , Zumba etc. Def some kind of boxfit too.
Now, weather... la Santa is in the north of Lanzarote and because of the hills ( very popular with cyclists) is can be a bit cooler and cloudy. The first time I went I hadn't realised and was expecting a full sun holiday, and the sun was blocked by clouds all week. But other times it has been blue sky and glorious sunshine! Even in a January we have had sun.
I haven't been on my own, but I would if had the opportunity! You can opt to share your apartment with someone of same sex which halves your cost. Lots of opportunities to chat to other people, and you can find partners for racquet sports by putting up a note in sports booking ( or asking the green team , who might be able to point you towards someone).
A big of entertainment in the eve if you fancy it, but can steer clear if not. Plenty of places to watch sport on tv!
Does that help?