Training by-product

In general I've been focusing on increasing my distances and keeping my speed roughly around 10min/mi. To some extent I thought time on Saturday was quicker because I was running in a (MUCH) larger group. Yet today I ran a 5k by myself and ended up doing it in 28:30 which included a long nasty hill and a bit of walking. I know at somepoint I will need to work specifically on speed but right now I'm still in the phase of wanting to increase my endurance.