On the plus side, you do have the older DCs to help!
I'm 32 weeks with DC4, my oldest is 7 years old, and I have a 5 year old and a 2 year old. My SPD kicks in around 10-15 weeks, meaning I can't walk as far. By 20 weeks it's essential journeys only and by 25 weeks it's definitely crutches time and wheelchairs/mobility scooters for malls and then it's downhill from there.
When my older ones help, it is great. However as they're older I have to do the school run for them. I had hoped before I got pregnant that some of the other mothers in my road would offer to take the older two on occasion but it's been quite clear they won't (they stride past me cheerfully saying hello each morning
- they know full well from many conversations how much pain I am in and so I'd figure they'd have offered if they were going to by now!) So for me the biggest issue is the school run and getting everyone ready in the morning.
In the house basically the living room has gates and is totally child-proofed, so most of the time I don't need to get up except for lunch/nappy changing. He's learned I don't pick him up any more, so he knows to go places I tell him like his changing mat etc. He climbs on me for cuddles just fine. I have remote controls for the TV to hand, he also enjoys playing on my iPad with me, and doing drawing at the table. The rest of the time he plays quite happily by himself (I know not all children will - DC2 preferred playmates) with his duplo, trains, soft toys etc.
I would find it very hard if said child was not yet mobile though - I think waiting until they are at least 18 months by the time the SPD kicks in so they can get themselves around without you needing to pick thems up is a good idea.
Of late DH will sometimes work from home (it's getting more frequent) especially on days when there are after school clubs and I'd have to make the journey three times. Today for instance there's an after school club then 45 minutes later, the school disco - FIVE journeys in a day. So DH is coming home early to work from home and do a few of those.
For the first time in our lives we've invested in a cleaner too. DH now does the picking up toys etc that I used to do, and the cleaners do the jobs he used to that don't involve personal knowledge of the house and where things go e.g. cleaning the kitchen, bathroom etc. I also got an extendable arm thing to pick up the odd thing off the floor, and regularly bribe the older ones to tidy up.
I won't lie - it is not easy, but it IS doable. It's "only" (feels like eternity, mind) for a few months and when you're living it, it is horrendous, but in the grand scheme of life it is worth it to have that new child.