Peroneus longus tendon injury?

I ran a 10km race on Sunday (my first).  I had trained and completed
it without any notable incidents.  There were no pains either after
the race or the next morning.  However, after about 5 minutes of being
on my feet the next day, I started with a pain in the area of my foot,
which I seem to have located as being the peroneus longus tendon
(about an inch to the front and below my ankle bone).  I have rested,
iced, elevated and put my foot in a compression bandage, but to no
avail.  Walking is quite painful, particularly in the morning.  The
one thing I have noticed is that it eases as the day goes on.  When I
go off for my lunch break and have to walk on it, it is very painful
at first but this subsides and whilst I am aware of it for the rest of
the day, it does not really hurt too much.  There is no swelling or
any obvious heat in the area.

I would like to know if someone could advise me - does it sound
serious, (ie ruptured or damaged) or does it just sound like
inflammation?  In either case, what advice do you have?

Thanks