ride uphill more. Get a singlespeed and aim to do it on that.
Go on – punish yourself!! I used to do load of hill repeats when I was in marathon (running) training mode – **** brutal, but it made the distance running sooo much easier.
I was, then, much more all round fit than I am now, but am now more bike fit than ever thanks to putting in the effort on a singlespeed over the past couple of years. Took about 3 weeks to stop thinking it was crackers and about 3 months to really notice the difference in fitness.
Or…
You could stick to what you’re good at & love doing? Training for an event can be good in that you have something to focus on but it could suck the fun out of your hobby.