As has been said, the recipe in question is an all-grain recipe. All-grain does take a certain amount of extra time and work, but for extra equipment, all you really need is a large strainer bag, as Beergolf says. Still, it's a significant leap in the difficulty from extract and specialty grains to a full mash. You may be ready to do it - it really isn't all
that hard, though it does end up taking all day for most people - but you'd have to decide for yourself if you want to make that jump.
You might want to look at
this post of mine for an overview of the all-grain brewing process. After that, try reading
How to Brew (preferably the print version, but the online book is still mostly valid) for the details of actually brewing from malt.
How large is the pot in question? While I am not familiar with Beer In A Bag myself, I would expect you would need at least a 32 quart boiler, preferably 40qt. or larger. The reason for the larger pot is because, after you mash the malt (that is, soak it in 148-158 deg. F water for 45-60 minutes), and sparge the grain bed (basically, rinse it with fresh water - with BIAB, as I understand it, you would simply lift the bag out of the wort and pour water through it), you'll end up with more wort than you want, at a lower gravity than you want, so you need to boil off some of the water. Fortunately, you already are using an exterior burner, which helps a lot; I know from experience that boil-offs on most stove tops are slow going.
If you don't feel like tackling all-grain, you still have some options, assuming the malt was pre-crushed (if it was from a kit, it probably was). You can do an extract and specialty malt version by substituting about 5.25 lbs of light DME for the Maris Otter base malt; put the specialty grains in a strainer bag, steep them in 155deg F water for 20 minutes before boiling, remove the grain bag, and then proceed as you would for an extract based recipe.
Alternately, you can do a partial mash, which would mean using part of the base malt (perhaps as much as half) in the steep, and steeping for 30 minutes instead of 20. You would need to substitute the rest of the base malt with extract, keeping the total ratio the same; so, if you use half of the base malt (4.25lbs.), you would use only 2.625 lbs. of extract; if you use 30% base malt (2.5lbs.), you would use 70% extract (3.675lbs); and so on (it doesn't have to be all that exact, but the more accurate you are the closer you'll be to the original recipe).
