Sinbad, are you doing the manchester marathon too? I'll be at that one.
If i'm reading your question correctly, do you mean is it possible to build up to a long run of 20miles in 10 weeks? I guess the answer depends on the shape you're currently in, and your capacity to train whilst avoiding injury.
You'd want to do a 2week or 3week taper, so your last long run, of 20 miles, would be on either 7th or 14th April. You would draw a straight line between now and that date, with the 20mile run on the 7th (or 14th) and your current long run from last weekend. Increase the mileage for each weekly long run, remembering to stagger it a bit, so you have some rest weeks each third or fourth week where you run maybe 75% of the previous week's distance. After hitting the peak mileage, you'd then taper for 2 or 3 weeks, ramping down to avoid injury and make sure you're fresh for race day.
so, just to illustrate, you might be on 10 miles for last Sunday, and each subsequent sunday between now and your peak would be something like:
12mi, 14mi, 16mi, 13mi, 17mi, 18mi, 15mi, 20mi, 16mi, 12mi, RACE
Now, ideally, you'd be able to ramp up to 20miles before that date and have about 3 runs of 20miles or so. But depending on where you're at now, that might not be possible. The suggestion above is just that, it's not prescriptive, you should always listen to your body and if adjust your training accordingly.
For example, i'm doing my first 20 miles this Sunday, then again in about 3 weeks time, then again on 7th April, and on the sundays in between this i'll be doing between 14 and 18, depending on the week, and whatever other training i've got planned for that week. But then i've been very slowly ramping up the miles for the past 7 weeks, from about 12 miles around xmas to now. Adding miles quickly increases the injury risk. So it is possible to get to a 20mile long run in 10 weeks, but it's a steep climb, depending where you're starting from.
And yes, it's normal for your legs to feel shaky, but if you feel pain then it's something to get looked at by a physio.