Some people fall behind because they've been injured; others because life outside running has been too busy. So what are the best tips in this situation?
Don't run on an injury or if you're ill or recovering from illness. Peer pressure (including the pressure from fundraising) to encourage one another to get active is all very good but it's not worth the risk of making an injury worse or extending the period for which you are ill.
Don't try to fit all the missing runs into the shortened time period or otherwise overtrain - you simply run the risk of getting injured or overtired.
The Eagles / EHM training plan for people who started their 12 week training at around 10k level allows for a full distance recce 2 weeks before race day. Other training plans don't expect a HM distance before race day - you may find that you can do the extra distance on race day because you're rested or with the excitement of race day.
Know where you are and increase your distance gradually.
Adjust your race day expectations to allow for the training you've missed and appreciate that you may struggle a bit more on race day because of the missed training.
Keep to your two week taper / peaking period (when you reduce the amount of training you do) - it’s generally accepted that it takes 10 days to 4 weeks to benefit from a run so running further in the final 10 days will only make you more tired before race day without providing any benefit. Undertrained but fresh on race day is better.