Pelion (Portaria in particular) is 340 km from Athens and about 220 km from Thessaloniki. To reach it with your own vehicle, you have to drive on Athens-Thessaloniki National Road and take the exit to Volos. Portaria is only 12 km away from the capital of Magnesia prefecture.
Alternatively, you can get there by one of the Magnesia KTEL buses departing both from Athens (Liosia station, tel. +30 210 8329585) and from Thessaloniki (tel. +30 2310 595424) to Volos. From there, there are other bus services on a daily basis from morning till evening to different areas of Pelion (Magnesia KTEL buses, tel. +30 24210 33253, http://ktelvolou.gr).