In the world of +Ingress , size doesn't necessarily matter. One great way to earn XP quickly is finding a group of portals relatively close together and hack them all until basically they burn out.
Here's the plan; hack it as often as possible over a few days, whether they are under the control of your faction or not. Build that inventory and keep track of the keys you get. then when you feel you have a fair amount of keys go in and build your links.
Be sure to to this strategically. In this case smaller is better. Link the portals which are closest to each other and build an interlocking series of small control fields. Yes, your control fields will yield very few Mind Units (MU) for your faction, but you are earning 313 AP for each link and 1250 AP for each control field.
You want to link the closer ones to ensure you do not block out other potential links and control fields. I've done it before and created too large of a link which prevents me from executing my master plan and now I can't create new links or fields.
No your control fields won't be these huge sexy multi-million MU control fields, but remember you earn the same weather it is 1MU or 10,000,000 MU. IMO until your are a level 7 or 8 don't worry too much about the global faction score, first worry about leveling up and getting yourself to the point where you can then worry about making the huge control fields.
Below is a local historical area where I hacked for a while to build the inventory of keys I needed to execute my plan. When all was said and done I created enough portals, links and control fields to earn over 130,000 AP in just one night. Not too shabby. Not to mention all the AP I earned building that inventory.