Everyone is different, and everyone will react differently. But I would urge caution - my initial recovery was quick, with no painkillers needed the day after and able to walk rather than limp within a few days. However the bit that took longest to recover was the deep bruising under the knee and around the inside of my leg, which made the knee tender to touch and unstable when load bearing.
All told, it took me about 12 weeks to be back to normal, although I probably could have started running after about 10 (I ended up getting over-cautious towards the end as I'd been a bit too keen at the start). Between 8 and 12 weeks I started cycling, gradually building over the weeks until the knee felt stable when sprinting out of the saddle.
The consultant who operated on me had said it would be two months until it was back to normal, which suggests I took a bit longer than average to recover.
As I said, everyone is different so you may be back after two weeks but don't be too concerned if it takes a bit longer as it will recover after a relatively short while.
My experience is the same as Jane's - I had a meniscal tear that didn't really affect day-to-day activities, it felt a bit weak and kneeling on it was uncomfortable but otherwise it was fine. However I couldn't run on it, and ended up giving myself a groin strain due to trying to minimise the weight on my knee whilst running.
The surgeon was a straight talker, and said it was my decision whether to have the operation, as I could have a decent "quality of life" without the operation. As running was important to me, I decide to have the Op and I'm glad I did. I'm now able to run without injury or discomfort.
I think medical practice has changed a lot over the last few years, with more focus on doing enough to ensure that patients can live comfortably rather than trying to fix every defect. You need to decide on an individual level whether the compromises you need to make to accommodate the tear are worth making.
I'm not sure what plan you are following, but its unusual to include a significant number of speed sessions within a marathon plan, as the focus is normally on endurance.
I think your on the right lines, make the aim of your first marathon to complete the distance (which is an achievement most of the general public will never accomplish) and then after that reconsider if you want to focus on long distance or increasing your speed over shorter distance.
Also you will probably find that your speed will improve anyway, as the marathon training will make a 10k feel like a 'short' run and you'll find it easier to complete the distance etc.
It does depend on how realistic your 3:15 time is. From memory, this is around the 7:20 pace for the marathon. Therefore your easy runs should be noticeably slower than this.
I used your target time when I last followed P&D and my easy runs were just over 8:00 pace, I based this on HR levels within the book. This would be a good starting point for you.
I like the P&D book, and I'm following it again for my next marathon, but one thing I've learnt this time is that it is written from an elite runners perspective. Be careful when adding percentages to pace and remember that they are thinking of around 6:00 race pace. This means that 10% is only 36 seconds slower than race pace, and 20% only 1'12" slower than race pace. Adding 1'12" to a 7:20 pace would give a 8:30 (ish) pace which is closer to what I find is the slow end of my usual easy runs. Going down to 8:55 pace is getting to the slow end of recovery heart rate zone for me.
This is a sample of one (i.e. me) so your experience may vary!