What was I thinking!

Hi all! I am (obviously) new here. I am also a new (aspiring) athlete. Last week my 19 yr old son said he wanted to do a Triathlon...and I suggest that we do it together as a mother/son thing! I think my brain turned off for a moment. I am a 43 yr old overweight woman who can barely run, I have to rest after 25 yds of swimming, and yesterday was the first time on a bike since my boys were very little! I have been working on losing weight (have lost 12 lbs in the last 3 weeks) for my and my husband's 25th anniversary, we are doing a vow renewal. Now I have made the commitment to my youngest son to do a triathlon with him in 2016. We were going to do the 2015 one but we aren't sure we will be able to come up with the entrance fee, not to mention equipment needed by then. Our first triathlon will be the Louisville Ironman, but that won't be our first race. We plan on starting off slowly and doing some 5k's and working our way up to a sprint triathlon this year and then the big one next year. Any tips would be greatly appreciated! Right now we are using an 11 wk training plan for a sprint. I am saving up the $$ to join the Y so I can get my swimming in, that should happen in January. Right now I am using dryland training for my swim days. Look forward to being part of this community!