I'm now a year and a half into taking up euphonium and switching from trumpet/french horn. At first I wondered how you guys found enough air to play even one phrase. Now I don't think about it much. I'm not sure when that happened. I also remember getting sore large facial muscles. Still happens sometimes. So for the OP, just keep working on it. It will happen. Oh, and by the way, within two months I could play anything thrown at me in community band. There is carry over from small brass instruments to larger ones.
Richard
King 1130 Flugabone
King 2280 Euphonium
King 10J Tuba
Conn 22B Trumpet