'Post Marathon Withdrawal Syndrome

I am not sure if this condition, Post Marathon Withdrawal Syndrom does exist, but if it doesn't, I was going to attempt to describe what I am feeling at this period and call it Post Marathon Withdrawal Syndrome.
I did a quick google check and I came up with an article by Doug Kurtis on www. Runmichigan.com in which he discuses the Post Marathon Depression postulated by Dr Suzanne McAllister.

From my understanding Post Marathon Depression is a feeling that marathon runners go through in the aftermath of a marathon. Because they have accomplished the marathon challenge they are left with the feeling; 'what next'. Marathons do not give them the same kicks as they did when they ran the marathon for the first time. So they go through this kind of depression. I would imagine that as a consequence of this, runners will be left with no appetite to compete in the next marathon

This is a bit different from what I am feeling. After having completed the marathon like maybe a few other first timers I crossed the finishing line nursing an injury. In my case the right knee. This means for me I have to take a longer rest from running than I would have allowed myself.

The idea of having to rest is really bothering me. The physiotherapist advised me to take some rest for a while. The challenge is to do exactly that; to rest and do no running. I feel there is something missing, 'something not right". Running had become enshrined into my regular routine. I feel much better everytime I run. So what am I going to substitute this with?

It is almost what psychiatrists would describe for alcohol or drug withdrawal. If they call the former, Alcohol withdrawal syndrome, then this could be the equivalence; ie Marathon Withdrawal Syndrome.
I suppose I might be exhibiting signs and symptoms of 'runnaholism'
Two weeks after running a marathon what do people do? Do they just sit and wait for the next marathon which may be several months, if not years away?
Physiotherapists have recommended that I engage in activities like swimming. Swimming does not demand a lot from my legs joints while at the same time I will get pretty much the same workout intensity level as running.
However swimming in general requires that one joints a swimming/gym club and also one has to do it according to the schedule of the club.
I am used to running along streets and beaches and parks for free and I can run whenever it suits me.
If I was able to go swimming in the Irish Seas everytime I feel like swimming that would be great. However I do not think it's a great idea especially this freezing time of the year. Besides I see noone else doing it, so I presume this is not a great idea.