I echo what everyone else says. It looks like a good first ultra. Is it doable? I would have thought so, but it will involve significant time commitment and is likely to hurt. You'll also be training though the winter, so brace yourself for long days out in foul weather. Only you can decide if that's worth doing.
I've done various fundraising events. It can be easier to fundraise for a marathon than an ultra, simply because everyone - runner or not - knows what a marathon is. Get used to saying, "I'm doing event which is..."
Finally, given that your base mileage is low and you're not usually running more than 5k at a time, realistically you're going to be walking significant chunks of this. That's fine. Lots of people, including me, do that. There's a thread here (spring marathon? training?) on run/walk techniques, which would be worth checking out.
Walking sounds easy, but walking 42 miles is an endeavour in itself. If you're not a walker, I would suggest that all day hikes form the backbone of your training. Its a relatively low impact way to improve base fitness, get used to time on your feet outdoors, and to practice nutrition/socks/footcare etc.
What WiB said holds true at the other end of the speed spectrum, too. My time for 50 miles of SDW is quite literally more than twice his*, but increasing your top speed increases your cruising speed wherever in the field those sit.
(*I like to think of this as getting full value for money by enjoying the course for as long as it's open.)