Here's a list of weight/height for elite American distance runners.
Name Weight Height Lbs/Inch
Abdi Abdirahman 130 71.00 1.83
Alan Culpepper 130 73.00 1.78
A. Famiglietti 127 69.00 1.84
Adam Goucher 138 70.00 1.97
Ryan Hall 130 70.00 1.86
Meb Keflezighi 127 67.00 1.90
Khalid Khannouchi 125 65.00 1.92
Bernard Legat 134 68.00 1.97
Dathan Ritzenhein 117 68.00 1.72
Galen Rupp 138 70.00 1.97
Brian Sell 140 71.00 1.97
Matt Tegenkamp 145 73.00 1.99
Averages 131.75 69.58 1.89
Looks like you are right in the middle of that range. Attempting to lose any weight seems pointless (and how would you do it, not eat enough?). Professional runners do lift weights, but low weight at high reps for muscle endurance. You don't want bulk which will slow you down. Higher mileage will help out your half times more than anything else.
As for a BMI of below 19 being unhealthy, well, Ritzenhein is at about 17, and I don't think an unhealthy person can put in a 2:10 marathon.
Edited: 04/05/2009 at 11:46