Walking is crucial I think - it's far less impact and it gets the joints moving. I've been trying to get more walking into my training routines generally, finding the time is the challenge but if you're feeling stiff and sore, it's the best option.
I walk between 6 and 10 miles most days with the dog, over the downs so plenty of hills and tend to go at a rate that has me working up a bit of a sweat, c15/min miles (which is harder than it sounds with hills on rough terrain!) and that's what's kept me going whilst I haven't been running for two years.
I'd not given any thought as to how the walking would help with general fitness etc,, which is why I was amazed that when I took my first few runs recently I was able to churn out a few miles without any real difficulty.
So, in short, I think walking can be amazingly beneficial to fill gaps when you're not able to run for any reason.
The only drawback is the time walking takes relative to running - as stupidly obvious as that is to say. Last week I ran one of my regular dog walks, with the dog, for the first time. It's a 5.6m loop which includes going to the top of Cissbury Ring. Running it took me 50 minutes, I walked it this morning and it took 35 minutes longer.