Glad im not alone here!
I also have a 5cm gap, i was so upset when i found out. When the physio came around to see me she told me to avoid sit ups at all costs, not to sit up straight from a lying down position but to roll to one side and sit up from there. She told me the baby is the heaviest thing i should be lifting right now (hard when theres stuff to do around the house though!)
She also give me some excersices to do -
1. Pelvic floor - do these all day as much as you can to really strengthen the muscles up and pull them together. I do them every time LO needs a feed so getting quite regular now and its only been 5 days but i can really feel them getting stronger. Avoid doing them when on the toilet!
2. Belly button pulls - lie flat on your back and place your feet flat on the floor pulling your knees up to the ceiling. Gently use the muscles to draw your belly button into your back, and hold for 10 seconds and release. Keep breathing steady a you do so. You may not be able to hold for the full 10 seconds at first so build it up and dont strain yourself too much. Do these as much as you can, you cannot over do them, the more you do, the more chance your stomach muscles have of pulling back together.
3. Wear a band! - I got given a belly wrap to wear just to hold my stomach together. (from the top of pubic bone to underneath my boobs) After labour my belly just flopped out and when I stood up I had difficulty in breathing and walking. The band made it so much easier to breathe and i was up and walking by myself within a cuple of days. Dont wear for long periods of time as your muscles can get lazy and rely on the band to hold them in too much. I wear for a couple of hours a day, usually when im most active.
One last thing - she also told me the first couple of weeks are cruicial for practicing these as its alot harder for them to draw back after a few weeks. My next check up is next monday so weill have to see if my efforts have paid off. I hope this helps