Hello Daniel
You remind me of someone I met doing the Weardale Way, he was a big lad!
He was struggling in the heat, constantly sitting down and drinking massive amounts of liquids.
For me, starting out, it was just about getting the correct walking boots/shoes that were comfortable. Then finding some local walking routes to see how far I could manage without stretching myself too far, just building up walking strength. Local Walking shops have very experienced staff who can offer advice on all the gear required. Weather proof gear will be important. Your size may be an issue, I'm size 14 walking boots, which were difficult to find to try on, most cheaper ones fell apart when wet. Decent footwear was the key for me, although I only do day walks, so I'm sure help will arrive from others!
I do a lot of preparation regarding route to be walked so have a good understanding of where I'm heading.
Hope all goes well and enjoy!