Very do-able and you don't HAVE to do mega hours of training. Most time consuming session is the long bike, so get up early at the weekend and get out the door at 6am and you can do 100 miles and be home for lunch.
Same boat as you when i started. Entered a OW std dist tri and could only swim about 100m before i had to take a rest. I think i did 7 swim sessions before the big day and was bricking it. I built up by doing sets of 50m and then resting 30 seconds. 25 sets, so 50 lengths total, then 30 sets and so on. Once i was doing some volume, the swim fitness built enabled me to swim further without resting. Managed the 1500m swim without resting, so it just needs some solid work. Agree lessons will help as well.