Get in a routine and have a plan, start small, so a short 1km run (whatever, pipe down back there), a few sprints; it can be trifling exercise at the start because we're just building momentum.
If you plan 4 days worth of exercise, and start with small sessions, you can soon scale them up as you find things getting easier.
And here's the key thing;
It's okay to quit if you can't be arsed or you're feeling not up for it on the day. As long as you move the day's session and do that exercise on another day. Early on, it is absolutely fine if you miss an entire session and only do 3 sessions.
I stress this as acceptable, because there's nothing more offputting early on than being too strict that you end up packing everything in. So, early on, you can sort of be flakey, as long as you're hitting most of the targets along the way.
Once you get a month or so into the training sessions though, you absolutely should be growing out of this mindset and into a more serious regimen, where you're nailing four sessions minimum and only ever rescheduling due to serious conflicts (i.e. The Wife's birthday).
Still, if that isn't motivation enough, some of the benefits to getting into exercise are;
1. Better fitness
2. Better refereeing thanks to your fitness.
3. Weight loss
4. Better sex life - I'll let you work out why.
5. Overall, better self-esteem from the benefits of training.
If you cannot get into it on your own, consider getting a personal trainer - the cost alone will make you turn up, and sometimes, having someone do the planning and admin work, as well as help you face-to-face is all that is needed to get you up to speed to train on your own.
If even that fails... You just have to accept that you will not get very far in refereeing (or with the fitness tests, unless you're just naturally fit), but hey, if you're content at the lower rungs of refereeing, that's okay too and there are lots of matches that will need refereeing.
realistically you should never have stopped excercising when the season was shutdown, and kept a base level of fitness.
Unfortunately, not always easy to do. I've been in two-minds on training since most of my training is gym-based. Myself and my partner have lung issues, which means trying to avoid all the people who are flouting the lockdown rules. We also lived in a crap area for running. Thankfully, we've moved house now, so can run with impunity, but I'm back to square 1 after basically two months of inactivity. Just have to train extra hard to get back and beyond I suppose.