Look at it this way: you're now part of the elite and vast club of people who've done just what you describe: lost and regained. Only you, my dear, didn't even regain half of that 70 lbs!
Your past efforts were NOT for nothing! Losing 70 lbs is AWESOME and hey - at least you didn't gain it all back plus more (like yours truly). So be proud of what you DID do!
And you KNOW you have the knowledge and fortitude to do it again. Like the others said, just focus on learning from the past as opposed to beating yourself up. Perfection has no place in this journey. The vast, vast majority have slipped a bit or a lot. What matters is striving to learn if this happens so eventually it will happen less, and just keep going!
You only lose out if you quit and anyone who has lost 70lbs and run a 5K sure doesn't sound like a quitter to me!
You can DO THIS!!