That is a pity. I've done the 50 once. I think the trouble is the event can't be started early enough in the morning and a huge time gap builds up between the full marathon and ultra runners doing their last 25 miles.
With asthma the important thing is to get it under control. If you've got symptoms (wheezing/tight chest/coughing when talking or laughing) you need to persuade your GP to give you a high strength preventer inhaler, use it regularly twice a day as prescribed, and then gradually reduce the dose to the minimum level you need to keep symptom free.
With the preventer inhaler (unlike the reliever) you never feel anything happening, in fact you wonder if anything is happening - you just have to trust that it is and keep taking it.
Always carry your reliever inhaler with you. It takes a short while to kick in so a good idea is take a couple of puffs 15 minutes before you go out.
Asthma may take a slight edge off your overall performance - I think it has with me - and you may find, like me, you can't race in winter (without making yourself very ill).
All the best. Paula Radcliffe has it, and Haile Gebrselassie, so there is still hope.
I have this, Muttley. It was hard to accept and I tried to ignore it for a year or two but you've got to come to terms with it and help yourself as much as possible.
For me it's only in winter when breathing in cold air. In the end it has meant I do no races between Nov-mid-March, but I continue training at a low intensity and only go out in the warmest part of the day.
An extremely good tip I can tell you is when you go out where a buff over your mouth and use it to breath through. It warms the air you breathe in. You will be utterly amazed at the difference, but it means you can't do high intensity/race pace work!
You get a similar but lesser effect by just wearing the buff around your neck.