I decided to run a marathon in May of this year (Colorado Marathon) around early November of 2014. So that's roughly 6 months to train for it. I had run a lot back in high school, and then stopped... I'm 44 (5'11", 160lbs) and high school was a really long time ago so I was essentially starting from nothing. I was in pretty good shape at the time that I decided to run - I was an avid cyclist (past tense, now I'm an avid runner... decided quickly that I couldn't do both and succeed well at either). But while I could ride a serious mountain cycling race, I had no recent experience running. Getting started was hard. You'd think cycling is legs and lungs and running is legs and lungs, but I found out that they are clearly different leg muscles quickly.
I set a goal time for my first marathon of sub-4 hours. Which is probably pretty aggressive, but, hey, it's a goal and I wanted it to be hard.
I went to a runners store, had them put me on a treadmill and we analyzed my gait and tried on a dozen pairs of super expensive shoes, picked my favorite and and I bought two pairs of the most expensive shoes that I've ever purchased. Then a bit later, I bought a Garmin 220 because it's really nice to have a watch and to load my times into Strava and Runkeeper and see how I'm doing.
I ran into some knee issues once I started getting over about 12 miles in a run some time in early March 2015, went to physical therapy, backed off the running for 2-3 weeks and then decided that I wasn't going to make it to marathon distances in time for May 2nd, so I moved to a half marathon. I finished the half in 1h42m, and signed up for the San Francisco Marathon on July 26, 2015. So in about 3 weeks from now. I'm still not sure if I can complete a marathon a marathon in sub 4 hours - I'm running a decent pace and decent lengths although my peak range isn't as far as it should be. But based on my pace and how I feel after running, I'm thinking I'm not too far off. We'll see. I'm going 20 tomorrow and then trying for 22 next week and I'll see how I run and how I feel and then I'll call it good enough and that will be the farthest I go until the 26th. I know people say that there's a psychological reason to complete a marathon before the real thing, but I'm hoping altitude (from here in CO at 5200ft. to sea level) and the crowd will pull me through the last 4 miles.
I didn't realize that the SF marathon is considered one of the harder (the hardest?) Boston qualifier due to the hills - although I'm feeling stupid for not realizing it until recently - and so that will impact my goal. But I'm committed so it's marathon or bust. If I bomb out in the SF Marathon, then I'll set my eye on the Colorado Marathon for next May (considered to be one of the easiest Boston Marathon qualifiers because it's mostly downhill) and try again next year. For the Colorado Marathon, I'm thinking my goal will be to qualify for the Boston Marathon... just to do it, not to actually go.
Starting out for me was very hard. Those first couple of months kind of sucked - I felt lousy after running, felt lousy while running, and my times were terrible. Then it got good for a while, and then once I got above 15 miles, it got hard again because various pains started cropping up in new and creative muscles that I'd never heard of. My best advice is to make a plan, make absolutely positively certain that your partner is bought into the goal (a marathon) and your plan to get there because the hours start to add up and then stick with that plan closely. On the partner thing, this is imporant... it was one thing for me to think about going out on 3+ hour run, it's another thing when my wife realized that I was leaving her with the kids for 3+ hours on a weekend to go running... and that this was going to happen virtually every weekend for several months. We had a few difficult conversations for a while there until my wife realized how important this was to me... and I shifted my running to early early morning (I get up around 5:30am to run) when it impacted the family less. Lastly, if you are like me and don't 't have a lot of running experience, let me tell you.. a marathon is a really long way to run. I had no idea what I was signing myself up for until recently... a marathon is far. When I'm out there and things start hurting and I still have a bunch of miles to go, I think "no wonder Pheidippides died doing it." and I've still never actually ran a real marathon yet. I am not sure how I'll do in SF in 3 weeks, but I feel like I'm in the best shape I've been in a long while.
Good luck, my friend.