Stick at it, right now they are treating you like it's day one week one, if you have some ronhill tracksters or bikesters to wear they will be much better in the mud and the wet than a pair of trackies, trackies soak up the water and rub like buggery, anything that gets baggy when wet is a pain to wear.
The other thing I would suggest is find yourself a circuit that gives you a mile and half run, maybe a few circuits of a park, aim for 15 minutes as a start point, thats a gentle jog or brisk walking pace and try and work your way down from there toward the ten minute mark, 15 isn't that hard and you will soon shave a few minutes off that time and feel really pleased with youself. Warm up properly and make sure you have proper footwear.
That will improve your CV fitness, which in turn makes the other side of it, the circuit training, get a lot easier.