Yup, WITP Grand Campain is the main MP attraction here, there are few (if any at all bar WITP:AE) other games of comparable strategic scale. But you need to do some thinking before investing youself into GC. If you are intimidated by the size, you can start with some of the smaller scenarios first, Guadalcanal being the classic. In fact if you are new to the game starting MP with some small scenario is a must.
Assuming this is a stock game GC you will be playing, no mods:
0. Look into must-read threads section IV.PBEM and AARs, lots of useul stuff there
tm.asp?m=1274014
Some of points I think as more important
1. Agree on a set of house rules to compensate for some of the game drawbacks so that the game be more enjoyable for both of you. You can look up for what rules players tend to use in AAR threads. Common ones are:
- restricting # of aircraft in a hex to prevent impenetrable ubercap / unstoppable 4-engine bomb menace
- restricting amphibious landings to base hexes only and no subs, to prevent easy mode landings / road sabotage / free recon
- restricting # of ships in ASW groups
- not using a number of well-known exploits
Many other rules are possible, depending on personal tastes
2. A vast majority of WITP players are control freaks =) and want to direct every unit personally to not let allied AI screw their brilliant plans. If both of you don't want to be bothered with land combat, a common practice is to PC-control China theatre for both Allies and Japan. PC control in other parts of the world is messing with air/naval assets and is a nasty thing generally, as it tends to do stupid things.
3. First turn is an exceptional one, so it's ok to spend a lot of time on it if you are playing for Japan. Allies have it easier. If you want to have it start in a historical way, you can choose scenario #15 and cut youself some slack wrt 1st turn prep. With scenario #16 on the other hand you will be able to develop your own invasion plan, pick your own targets and pace as Japan paying with time spent on 1st turn in return