Multi-sport Strava Challenges
I've just completed all three running challenges for January (Prove It 100km, 10k, 1/2 marathon) and both cycling challenges (Prove it 600km, 130km Gran Fondo) but most months it will be a struggle to complete them all, not least because (say for February) cycling 1250km AND running 160km will take a huge chunk of time (say 50hrs on the bike and 13hrs on the road). That's a lot for anyone's schedule and at more than 2hrs a day throughout February would eat into most people's personal/ family life. And your badges don't currently do anything to recognise simultaneous completion of multiple challenges (like the 20+13 running AND 500km cycling over the Christmas break).
Why not allow mixed challenges? Say one that allowed you to meet the February target by completing 25% of the running challenge and 75% of the cycling challenge, or give some credit for swimming or other activities as well? That would offer more scope for people who cover mixed activities, and you could have special badges for the rare few who do spend 63 hours completing 100% of both.
You could actually go further and create a bespoke bucket list each month: say for March you might have to i) run 10k; ii) cycle 100k; iii) run a climb of 250m; iv) cycle a climb of 1000m; or add swimming and other disciplines and say you need to complete 4 out of 5 to get the overall badge. Maybe with bronze, silver and gold for each type of activity? You could create a different combination each month (seems like everyone wants more variety than simple time-based or distance-based badges) and this would also encourage people out of their comfort zones - maybe they'd find it easy to complete 3 out of the 4 but would have to stretch a bit to cover their weaker discipline?