This is a long post with lots of probably unnecessary details so for the tl;dr see the question at the bottom.
Hey everyone, I've run shorter distances (my favorite race is the 5k) all my life simply because I don't have the patience to do really long training runs every day, but recently my friend approached me to do a half marathon. I had about 1 month to train for it, I simply added a 10 mile run every Saturday to replace the hard 3-miler I usually do and managed to finish in 1:45, which is worse than my 20-ish minute 5ks but given only 1 month of training for the half vs 10 years of running 2 or 3 5ks a year respectable (yes I am slow, deal with it).
I usually run 1 or 2 5ks a year with my friends, but I always make sure to do one with my dad. He's turning 50 this year, and while he's slower than I am by a fair amount I'll usually stay with him for most of the race and treat it as a "fun run." Yesterday, however, he sent me an email. He saw some pictures of my half marathon and decided that since he's never run a marathon and he won't be able to run in a few years, he wanted to cross it off his bucket list with me. He's been training for it and given his dedication I'm sure he'll be able to finish, so I'm not worried about him.
My question to you is: how much harder is a full marathon than a half marathon? Would I need any extra training besides what I did for the half marathon? I don't have any particular desire to do it any faster than 3:30 (i.e. I don't want to do any extreme training), but I do want to be faster than 4:00 since any slower than that for a 28 year-old is prettttttty slow. I probably wouldn't stay with my dad except for the first few miles, because I do want my time to be respectable for someone my age.
tl;dr I ran a half marathon in 1:45 by having my only long distance training a 10-mile run once a week (running faster and shorter other days). If I want to run a full in between 3:30 and 4:00, how much additional training would I need to do besides what I did for the half?
Thanks