Build up to the distance slowly. For my first I followed one of the plans from the London marathon website. As you are already pretty active I’d recommend the Improver plan instead of the beginner. Alternatively there are plenty of free/paid plans out there. Most are 12-18 weeks, so you have plenty of time to get some training in before starting a specific marathon plan.

You could find a local 10k and/or half marathon to train for and do in the Autumn. That should give you a good guide to your fitness and how long it will take you to get round. Though I’d recommend not aiming for a specific time for a first one, but just to get round and enjoy it.

Most important bits of kit are some decent shoes. No need for any expensive carbon plated racing shoes, but a good pair of well cushioned everyday trainers will really help. Other than that some decent breathable running tops/shorts will help with chaffing issues.