I am a member of Black Country Tri and I help out & marshall each year at the Longest Day Tri, and believe me we see all shapes & sizes crossing the finishing line. Obviously you will need to train properly for the event, but from my own IM racing experience and observations at TLD the most important ingredient is the desire to succeed, and you already have that.
The Longest Day is the ideal event for 1st timers - the swim is in a lake, the bike course has no major hills, and the run is flat & shaded - and you get plenty of support from the race team and supporters.
Go for it, you won't regret it!