Welcome back to the world of horses, I'm sure you won't regret it (though your muscles and bank balance may say otherwise). I'm sure you'll regain the knack in no time having ridden regularly as a child.
I definitely think some lessons at a good riding school are the best way to start, after that the world's your oyster, riding holidays, riding clubs, a loan/share or even your own horse or pony are all options!
The most important thing is to pick a good riding school for your initial lessons. Start with the British Horse Society (BHS)'s list of approved schools on their website. Ideally you want a school that is large enough to cater for adult beginners, but not so large you just feel like a number to them. Some smaller schools mainly deal with children so will only have a few horses suitable for adult novices, and/or will stick you in with the tots for your first classes, not good! A place that runs an specific adults returning to riding course as Constance describes is ideal as you can make friends with your group this way too. But otherwise a 'package' of private lessons starting off on a lunge or lead rein and building up to walk, trot and canter independently and then perhaps progressing to group classes would be fine. If you don't mind posting where in the country you are posters here might have a recommendation for you?
I would highly recommend you visit a few schools before making your selection. Personally I don't mind if the place isn't super-smart, slightly muddy ponies, scruffy tack and flaky paintwork are par for the course at many excellent schools, but the animals should look happy and well-cared for, and the tack and general condition of the yard should be safe. Try and observe a few lessons and see how the instructors interact with the clients - it's important to find an instructor you get along with and not all instructors, however well qualified, have the gift of teaching beginners!
All the best with it OP...