Becky, yay for the pool! And your 1500 after not swimming for months is still waaaay faster than my 1500.
Midwife, is the swim in a pool, a lake, or the ocean? That can make a big difference in time. Swimming in a pool is faster and less stressful and prone to panicking. One of the tri's in my area does a 400 meter swim in a pool, and the last person to finish the swim usually comes in around 20-24 minutes (the first person comes in at 5 minutes!). For me at least, swimming in open water instead adds about 3 minutes to my total time.
In any case I'd guess you could probably finish 400 meters in about 15-18 minutes if you're a slowpoke like me, and much faster if you have a strong upper body.
Megan, good job getting rid of the turkey weight!