Lymm Triathlon

I got my time yesterday for the Lymm Triathlon, I told them I am just learning to swim so I am not sure how long it would take me, so they have put me down for 11 minutes!! (with the fastest swimmers). I have emailed them and told them that there is no way I can do the swim in that time, it will take me at least 30 minutes. They will not change my time saying it won't be a problem. But as there are 5 swimmers per lane at a time, it is going to be a problem for those swimming with me. It means that I will have to keep stopping to let the faster swimmers pass me, and I have worked out that I will still be on the cycle when they are presenting the winners!!
This is my first ever triathlon and my only wish was to finish, but I feel under pressure and its starting to stress me out.