You can run in boots (I did in the Army) but my recommendation would be get some running shoes, the majority start around £30 get them in black and if you quit they'll be fine for everyday use.The last thing you want if you start enjoying your run is to get an injury that keeps you off your feet for a few weeks.
Swimming is fantastic for off running days, practically no chance of injury (drowning however is a possibility which will hamper your training) I try and get a few lengths, alternating from breast 1 length back 1 length then speed with front crawl 2 lengths and repeat..I generally find the next day I can run easily up to 10k with no issues on my legs, currently training for Bolton HM this Sunday and I believe the cross training has helped with my speed
Couple or more thoughts for you ..The brain gives up long before the body needs too, don't think about the miles before you, but about the miles you have completed, the quickest way to the finish line is to run..
And be in the moment, concentrate on what you are doing, don't worry about what is ahead ...'are you going to need to walk' doesn't matter until you 'need' to walk so don't think about it, if you do need to focus on something start imagining the finish line or that nice long soak in the bath afterwards...distance running is all in the mind
I have the Liverpool Half Marathon on Sunday, only got up to 10 miles in training before I got injured, so I will be just hobbling around but I know I can finish, and so will you. Good luck