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!
Have a google for the 'mcmillan pace calculator' to give an indication of what you could achieve at half marathon distance based on your 10k time. Your current 10k time translates to a 1:40+ HM time.
This assumes you can maintain an equivalent level of performance over the longer distance, which typically is a struggle for most people when they double the distance. As an ex-footballer you will probably struggle with endurance as it relies on short sprints rather than sustained, paced running.
For your first HM the target should be finishing (as any time will be a PB) and then you can try to get faster at subsequent attempts. If your using a training plan, you'd be better of using 1:40 as a target time for setting training paces etc.