How am I gonna manage this ?

I think I need help.

I've entered this http://www.oetztaler-radmarathon.com/ due to a mid-life crisis (40 this year) and because my mate lives near to it.

I cycle a bit over winter which seems to be almost over and do roughly 3 spin classes a week.
In the last month I've been doing a spin class or 1hour on a spin bike almost daily and dropped 1/2 stone to 13st 2lb (I'm 5ft 8").

My normal Summer cycling week is as follows:
I commute 18 miles each way approximately 3 days a week.
Wednesday I have time for a 2hr ride and some eveings wife and kids permitting.
Weekends I can get out for 50miles each day although the wife knows I am going to have to do bigger rides at least 1 day a weekend as the summer draws on.

I am on the Dragon Ride Gran Fondo June 6th and doing London to Brighton and back again a few weeks after.
I have a family holiday in Majorca for 12 days at the end of June and again at the start of August and ride most days when there.

I'm trying to get to 12 stone for the first time in at least 15 years which will help no end but other than that and riding a lot I don't have much idea about what I should be doing.

How much more do you think I should be doing ?
Is there a set number of miles I should be doing each week ?
Would there be much benefit in getting an on-line coach draw up a 3 month plan ? (Seen a few for about £100 to £175)
Or am I reading comics thinking I can manage this ?

Oh and my bikes gone back to Focus for a few weeks due to a warranty issue !

Thanks