If on the road, don't turn left to stay on Mass Ave; go straight onto Bedford Rd. Bedford Rd becomes Great Rd Bedford, which you take to the Rt 4 / Rt 225 split.
If on the path, take it to the end, then take a right and follow that road up to the light at Great Rd. Take a left and proceed to the Rt 225 / rt4 split.
Bear right on Rt 4. Follow a few miles to the light with the Billerica Fire Station. Take a left (onto Treble Cove?), then a very quick right onto the former North Rd (now it's called East St or West St, but the sign still mentions North Rd). North / East or North / West, whatever, goes straight into the state park. YOU ARE THERE.
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There are better routes through the northern part of Lexington and in Bedford if you like hills. From Lexington Center you can bear right on Hancock (or get off the path directly onto Hancock).
Then right at the rotary onto Burlington St / Grove St (never clearly marked).
Over I 95 / 128 on Grove St, up the first good hill, road becomes Page Rd, down the hill to Rt 62.
Straight onto 62 West, then quickly Straight off 62 West still on Page Rd.
Page Rd becomes Pine Hill Rd for a short steep climb, then the road ends at Rt 4. Take a right on Rt 4 and follow the directions above.
Have fun!
Dave Adams