How do you come up with a race training plan?

Hi everyone!

What are the best programs out there to follow when preparing for a race? I've spent a lot of time reviewing Hal Higdon, Runner's World, Nike Run Club and have been tailoring my own plan that follows the 80/20 rule, increasing weekly mileage by no more than 10% each week and including strength training, but overall I lack confidence in the plan I'm coming up with for myself because I'm not that experienced.

I ran my first half this year, and my 2025 plan includes running that same half (hopefully PR'ing it) and then running my first marathon 20 weeks later. I have plenty of time to train as the half is in June, but want some advice on coming up with training plans. Are any of the paid apps worth it? TIA!