Is my sub-3.45 marathon goal realistic?

I am trying to assess whether my marathon goals are realistic with just over 4 months until the Paris marathon, and hoping some runners might be able to share their training performance history so I can assess if I can get to 12km/hr = 5min k's pace  for Paris Marathon?  My target is a sub-3.45.

I am currently only training on the treadmill, and am starting my long runs outdoors this weekend. 

I have currently been doing 2 x 10km (55mins) and 2 x 6km (33mins) per week and about to build more onto that.  So currently holding 11km/hr = 5.30 min k's over short distances (but again this is just on the treadmill).

I have done some interval training, eg 5 x (2mins of 11.5km/hr, 1min of 12.5km/hr and 1min of 13.5km/hr) etc and intend on improving on and working on intervals once I get my base up.

I'm also doing some 5k events in the hope of improving speed.

I ran my first marathon 3 years ago in 3.55 (and I trained for all of that outdoors) but have done little since then.

I am also swimming at least 2 days a week 1 - 2 kms each (sometimes straight off my 6 or 10km runs) And, I am doing 1 - 2 45min spin classes per week or otherwise one of these is a 50km road ride.

Once I get my base up and am comfortably building on my long runs by 15 mins or so a week, and then also with interval and hill training during the weekdays, can I get to 12km/hr = 5min k's pace in time for Paris Marathon on 6th April to achieve my target of sub-3.45?

Would appreciate training history or advice from anyone who has cut 30secs per km off their time, and over long distances, in 4 months!

You may benefit from dropping the swimming and spinning, which will tire you out without contributing anything useful to your running-specific training.

Good luck  

Get off the treadmill and get outside on the roads, there's a big difference between running on the supported tready surface and the rock-hard tarmac. Ingrid Kristiansen ran a world record after doing all her winter training on a treadmill, but she is Norwegian and a superwoman, ordinary mortals find it a lot harder. With 4 months still to go, you need to start increasing your long runs now, and I would find a half-marathon test race to do to check on your pace. Good luck.