I’m a cyclist and returning to racing after a few years off. I train 5-6 days a week.
I have a fairly specific programme I work to geared towards helping me achieve my racing goals... which you’d expect with someone who races! What I would advise is you pick something you enjoy, pick a goal - eg it might be riding a particular sportive or doing a half marathon or a sprint triathlon, then draw up a plan to work towards it. There’s a lot of help online with websites offering programmes for free. It doesn’t have to be an event, like I mentioned above it could be riding a century or running a 10k in a particular time. Or just cycling x amount of hours a week and enjoying it. Likewise it could be go to gym for three one hour slots a week.
One thing I’ve found (even when I didn’t race) is that having a plan and a goal is really important (over the past few years it was literally just: ride bike 10 hours per week, ride 500 hours over a year). Otherwise you get bored, can’t measure your progress and there’s nothing to pull you through those low/ no motivation days. Even when I really can’t be bloody bothered and my motivation has got up and gone, I do what I’ve planned, because I know it’s part of a bigger picture and I’m working toward something.
I track everything I do via a Garmin it all gets uploaded to Training Peaks and Strava. It’s so easy/ cheap nowadays with wearable devices like Fitbit and apps like Strava to track. Whatever you pick I’d really advise you to record it. Even if it’s just a paper diary where you write what you did. Eg cycled 20 miles, 2,000 foot climbing in 2 hours. Average speed. Average heart rate. That way you can look back and see your progress. That can be so motivating.
I also think nowadays it’s quite important to be adaptable or pick a sport that isn’t reliant on others or a facility. By ‘others’ I mean taking up a team sport like football. By facility I mean a gym or a pool. IF there was ever another lockdown, or restrictions, or you even just have to self isolate for 2 weeks - you don’t want it to cause you to jack it all in! I’ve seen so many threads on mumsnet where people have just given up as they can’t go to the gym or park run was off. So if you pick ‘gym’ attendance as your thing, maybe ensure you have equipment at home to replicate (you can buy stuff like resistance bands really cheap). I know in the winter, it’ll often be too icy for me to ride when I have a ‘window’ so I have a turbo trainer (so I can ride indoors) or I’ll ride off road on my MTB. Consequently I don’t miss out! If racing gets withdrawn again, I’ll race on Zwift (online platform). Anyway. Just think about a workable plan b so you don’t undo your hard work. Part of being a successful, committed, lifelong exerciser is about being adaptable. Weather it’s working back from injury or dealing with change.
Maybe your first few weeks should be about trying different sports/ activities and see how they work for you. Try a gym. Try a pool. Go for a long walk. Try a bike ride. Try a class. Then pick something you loved most. Or even a variety. I ‘swim’ (literally just a 10 minute dunk in a river) several times a week. I’m not a swimmer I get no training benefit, but I love it, it’s great for my mental health and immune system. I also run occasionally, again because I enjoy it. I mix up my cycling although I’m a roadie, I’ll mountain bike. I think that’s the other important thing about exercise (apart from the enjoyment) is that it benefits you, it’s not just about physical fitness - but the immune system boost, or that mental health boost or that time out to yourself. I love racing for many reasons, but I particularly love how it gives me something worth while to do. It’s doing something for me. The fitness is a bonus, likewise Health improvement.