If the northern terminus at Monument 78 is unreachable thru hikers will often set their sights on some other point on the US/Canada border.

I recommend Monument 90 as an alternate, but some people do not want to skip ahead that much. The good part about M47/M49 is that its quite beautiful north of Baker Lake, but the 73 miles of road walking south of there not so much.

Directions to Monument 47 (or 49)

Here I'm assuming you have a GPS map on your phone to ensure you are on this route. Driving: Highway 20 eastbound to near Raser State Park. Turn left (north) onto Baker Lake Road and drive 20.2 miles to Park Creek Campground/Swift Creek Campgound. (Alternately, you could jump ahead by only going 18.3 miles on Baker Lake Road, then turning left onto dirt road FR-1130 to the Swift Creek Trailhead which bypasses 2.6 miles and 566 ft. of the trail.)

(0.0 miles) Starting at the end of the north loop at the Park Creek Campground, the first 2.6 miles of the PN Trail from Park Creek Campground is an old forest road (1144)).
(2.6 miles) Near Baker Hot Springs (really just a warm mud puddle) the trail becomes Swift Creek Trail #607 and continues the gorgeous climb up the Switft Creek Valley.
(9.6 m) The trail will T-off with the Lake Ann Trail #600. Turn left (west) to continue up the west fork of the Swift Creek valley. The trail will get steeper and steeper until you close in on the Lake Ann Trailhead.
(12.2 m) Cross the paved road 542 and immediately take a right turn to begin the desent into the also-gorgeous chain lakes valley. I recommend following the west bank of the lakes, but if you are in a hurry you can stay on the PNT which hugs the west side of the highway.
(13.4 m via the short route) Once you are at Sunrise Lake and the bottom of Charilift #1 you must get a ride from a driver. Hwy 542 is not a walking road. It has no shoulder, no rails, and is mostly a rock wall on the uphill side and a cliff on the downhill side.
(22 m) You only need a ride for 8.6 miles down into the Nooksack RIver valey. Just past the Silver Fir Campground there is a dirt road on the left, FR-3060 that leads to the Welcome Pass Trailhead. Get droped off there and walk the 2/3 mile to the trailhead, or perhaps your driver is willing to take you all the way there.
(22.7 m/ 0.0 reset) From the Welcome Pass Trailhead, take the High Divide Trail #630.
(2.8 m) After 2.8 grueling miles gaining 3,025 ft you will reach Welcome Pass. From here you will enjoy a more leisurely ridge walk with gorgeous views to the north and the south.
(7.7 m) At Excelsior Pass (where there is a toilet) you will get onto Trail #625 northbound. Just beyond the pass there is a small lake with campsites. There is another small lake with campsites further down the valley at 9.6 m (Damfino Lakes).
(9.8 m) Veer right to get on Canyon Ridge Trail #689.
(12.5) If you are feeling adventurous, you can pursue Monument 49 instead of Monument 47. That will require some wayfinding. Monument 49 is less than 2 miles NE from here following the ridgeline.
(19.5) Assuming you continued on toward Monument 47 on Canyon Ridge Trail, right after one large switchback you will meet and old dirt road that runs due north along the ridge line. That is your path to Monument 47.
To shorten your exit you can just walk south on FR-3140 until you find a ride out.