Are you sure the transmission is actually going into first? Perhaps the clutch disks are stuck together and the clutch isn't releasing; the shift lever may be moving but not quite far enough to engage first. Shut the motor off while you think it's in first and try to push the bike in that state (without applying clutch lever). If it's in first and everything is kosher the motor will fight your progress. If it pushes easily it may actually still be in neutral; in that case you should be able to get it into first by applying pressure to the shift lever while pushing the bike (again, without applying the clutch lever). As it slips into first the bike should stop moving.
If that's how it pans out, then I'd say your clutch disks are stuck together. To free them, with the transmission fully in first, try pushing the bike (now WITH the clutch lever applied). Alternatively, you may be able to free them by starting the motor while in first with the clutch lever applied. You'll need to be careful of the bike taking off on you (use the brakes).
If the bike had a center stand, I'd say put it on the center stand, start it in first, and with the clutch lever engaged, use the rear brake to free the clutch disks.
When I bought my 440 LTD it had sat idle for at least 10 years. The clutch disks were stuck firmly together and it took quite a bit of pushing back and forth as described to free them. Once they were freed up the clutch was fine, though.
800 Club #056
2005 VN800A (Blue)
1980 KZ 440A LTD (for sale)