DRG THE LANDING ROLL, THE ACFT RAN OFF THE DEPARTURE END OF THE WET RWY & WENT THRU A FENCE & DITCH BEFORE COMING TO A STOP ON A ROADWAY. THE WX CONDS AT THE TIME OF THE ACDNT WERE DESCRIBED AS 'MUCKY,' WITH LOW CEILINGS (200' TO 400'), LOW VISIBILITIES (1 TO 1-1/2 MI), & A STEADY RAIN. A WITNESS, WHO SAW THE ACFT ON SHORT FINAL APCH FOR THE RWY, RPRTD THAT IT APPEARED TO BE TOO FAST. HE ESTIMATED THE ACFT TOUCHED DOWN APRX 600' FROM THE DEPARTURE END OF THE 4000' RWY. THE 63 YR OLD VFR PLT & THE 86 YR OLD ACFT OWNER TOLD WITNESSES THAT THEY DID NOT HAVE ENOUGH FUEL TO GO AROUND. AN INSPN OF THE FUEL TANKS REVEALED ABOUT 5 GAL OF FUEL IN THE LEFT TANK, WHILE THE RIGHT TANK WAS ALMOST EMPTY. THE ACFT OWNER/RIGHT SEAT PASSENGER WAS ALSO A RATED PLT, BUT SHE HAD OPEN HEART SURGERY IN LATE 1988 & NO LONGER HELD A VALID MED CERT. APRX 39 MI ESE AT MILWAUKEE, THE 1650 CDT WX WAS IN PART: PARTIAL OBSCURATION, 3500' BROKEN, VISIBILITY 1.5 MI HAZE & LIGHT RAIN, WIND 070 DEG AT 8 KTS.