This is a subject that is often debated and the truth is that there is no right answer, it really depends on the individual and their goals and situation.
Personally I always preferred training with a group, especially when I was training for marathons. It can be soul destroying doing a 20 miler on your own, I found it much better to have somebody to run with, if nothing else it took my mind off the pain I was going through.
Also I found it better to run with others that were slightly better than me, as it definitely improved my speed and endurance over time. Also I found it better when I joined a club, in my case it was Sparkhill Harriers, ib Hall Green, Birmingham. We used to train Tues, Thurs evenings and Sunday mornings. Most sessions were on the road but we also did track sessions (which were killers). I was definitely in the best condition in those days.
But, as I said, it depends on the individual situation, I have also trained for races on my own, and, although it's hard work, I have always been competitive even with myself, but I always kept a runners diary, which I linked to my plan, this enabled me to look at previous weeks and review my progress and amend my future weeks plan accordingly.
As I say it's up to you, but if anyone wants advice please post a question on the forum, or email me at firstname.lastname@example.org or ring me 07970 730790.
Don't forget we offer a personalised running plan service for £30 and there are generic plans available in the training plan shop too.