Towards Velestino (then, north to Thessaloniki or south to Athens): take the Larisis street, until you reach the junction with the peripheral. (city bus going that direction to be confirmed). From there it's easy to catch somebody coming from Volos or the Pilio mountain. On business hours they only go to the nearby factories, but on Sunday afternoon there's many people going back home after the weekend, so it's the best time to catch a long ride to [Thessaloniki] or [Athens]. If the driver doesn't go to your direction, ask him to drop you at the Velestino intersection, which is much easier. Velestino can also be reached by train from the center of Volos, with just 1,3€, then the intersection is just 600m away
Towards N. Agxialos (then south to Athens) : This road is slow and passing through many villages. Although it's shorter in distance, it's longer in time. You will easily get rides to the next village, and eventually somebody going to Athens.
Out of topic: train to Larisa every 2 hours costs 3,6€ at the desk (3,2€ online), and to Thessaloniki at 8:30am 12€ at the desk (9,1€ online). Maybe you want to avoid hitch-hiking, if this cost is small to you.