MM
Speaking as one who would have answered you on 243.0/121.5, I can only reiterate the old adage "Aviate - Navigate - Communicate". Bob V. is absolutely correct.
Yes the 'MAYDAY' call is essential to either, get you on the ground without hindrance, or, get the emergency services to your field of choice ASAP.
As said above, no sensible aviator is going to criticise you for calling MAYDAY and subsequently downgrading it as the situation resolves itself - least of all the guys on the ground doing their best to assist!