Yes, swimming will be tough if you've not swum before especially if your technique is poor.
As mentioned in a previous post, traditionally swimming has been about yards/laps/distance, even for sprinters.
I questioned this years ago as a lazy sprinter - why did I need to be swimming steady state laps for miles and miles each week? And was thrown out of the national squad!
The advice in the previous post is sound enough but consider also the various discussions found elsewhere on this board regarding low intensity vs high intensity energy systems work.
In my experience sprint/interval work is better for fat loss - it is my preference also,the thought of having to cover distances makes me weep, so perhaps my anecdotal evidence is biased!
Interval work is also completed far quicker.
10x25m swim sprints on 30seconds will take you roughly 9 minutes to complete (20seconds swim + 30 second rest).
Consider also that the energy system requirements will be similar to that of a 100m running sprint (a world class swimmer will complete 25m in approx 10sec, a world class sprint runner 100m is 10sec) so if that gives you an indication of the effort required it may give you an indication of where to start.
Hope that helps.