Duathlon advice needed

I put this on the 'old' forum and didn't get a huge amount of feedback, but having read the daily session stuff, it would appear there are a few more triathletes than before.
Given I can't swim, duathlons are the closest I'll come to ironman, but I've not done one before.
Here's the rub : I've only got a mountain bike which I don't ride hell of a lot to be honest and it's not one of the top of the range sort.However it's good enough for 20-30k required for shorter distance events.
(One locally is 6k run/20k bike/5k run).
I don't know if this is something I'm going to do a lot of , but will the bike be up to scratch for an event of this sort? My running speed should ensure I'm not left behind too much - although even if I'm last I'm not too bothered.
If I get the bug, obviously I'll upgrade the bike,but it doesn't seem worth the cost for my debut.
Sorry to waffle on, but any advice appreciated.

Dustin, assuming that it's on roads then over a distance of 20k I would expect to lose between 7 or 8 minutes using my mountain bike with smooth tyres or nearer 10 minutes with knobbly tyres. It does make a big difference, but if you're doing it for the experience it won't matter 'cause you'll probably find some other people doing exactly the same.

Where is the event and when does it take place?

Keep an eye on events listings and you will spot some Duathlons are off-road – here you need to use a MTB – just a thought.

And I agree with Drew’s estimates

Will

Dustin,

As long as the bike is roadworthy then you will have no problems. I am sure that you will not be the only one riding a mountain bike. As Drew points out you will lose a bit of time, but what does that matter? I am not sure if you have any specialised pedal system on your bike, but if you don't it may be wise to invest in a pair of toe-clips (think you can get some called Powertaps or something like that) so you can make full use of your pedal stroke. At least you won't have to change shoes so you will save time in transition :) Other than that pump up your tyres to the maximum pressure and go for it.

Gareth

Cheers guys.
The event is in Fleet (Hants) on Bank Holiday Monday, but given I'm doing Burnham Beeches/Great South/Windsor & am in the middle of my 'summer' training, then I'm not sure that I'll do it. However they seem to stage more than 1 a year so it's definitely something I'm keen on doing.
I assumed the bike would be slower,but over the 2 runs I'd like to think I'd make some time up.
Thanks for the timing tips,and kit advice and WW I'll keep an eye out for off roaders.

Thanks again.