As someone who ballooned to 108kg a little over a year ago (despite cycling 3 times a week and posting three century plus rides in the same year) I joined a gym (more accurately went to weight training with a PT) and without much difficulty got down to just over 92kg. I think the key to the success was the PT as opposed to just the gym and crucially (it turns out) it is based solely on weigh training.
5 days ago I picked up a copy of the excellent 'The Midlife Cyclist' by Phil Cavell (bike fitter and designer to a host of top level cyclists) off the back of some positive reviews from the press it (covertly) blames for most of what we do wrong. With much contribution from top drawer cardiologists, osteopaths, nutritionists, etc who deal with top and amateur level sportspeople and their issues. The whole book is predicated on the facts that we aren't yet evolved to be alive past 40 years old, much less athletes and certainly not to ride bikes and science is only now, during our generation catching up with this, because there hasn't been a generation doing what we're doing.
I was overjoyed to find out that weights are one of the single best things you can do for cycling, but it's the other stuff that was as much of a revelation to read. I really can't recommend it enough as a read BEFORE you plan how to shift some timber. Hint: it won't be rowing or swimming.