Okay so I don’t actually like the word ‘Brag’ as it sounds big-headed but if you struggle to commit to things then using peer pressure is often the best way to keep you on track.
Once you’ve entered the event, let your friends and family be part of your training by sharing every step of your programme with them. Use apps like Strava to measure your distances and times and post it out for the world to see.
It will be amazing how motivating it is to have lots of people behind your goal, and not just you.
This could also be the perfect opportunity to raise money for a charity close to your heart. Fundraising pages like Justgiving makes this so easy to do