I'm so tired of this I have to change now

to keep a long story short. I'm a 21 year old women who's 5'7 and 290 pounds. I'm so tired of myself at this point so I'm signing up to run in December. I'm paying the money and running a half marathon. Well walk/run. I've got an app for calorie counting and running so I can slowly build up my endurance. My boyfriend wants to propose soon and I hit rock bottom thinking of how much I'd hate my engagement photos if I didn't do something about my body. But I can! I was an athlete in high school and I can fucking do it again! Any tips on running/ dieting would be appreciated!!!!