Just after some friendly advise

I set off on a world tour in April and I've had a massive change of heart on the route, but before commiting to booking a couple of flights, I'd like to know anything I can with regards to entering North America & Canada on a British passport overland. (I'll also be scanning various other visa related websites for info too).

The route in a nutshell is 3 months around Scandinavia/Europe. Fly from Spain to Argentina and then cycle north to Vancouver, turn right and head towards Toronto to catch a flight to Japan to then bike south to Australia and finally if my budget permits and I haven't got bored by then, then maybe cycle back home to England.

So my questions would be.

1) I understand that, being a Brit, I get a 90 day visa, but...is it as simple as me just entering the US on my bike via Mexico and thats that, or do I need proof that I intend to leave?

2) if so, would my Toronto to Tokyo plane ticket be sufficient enough to prove this?

3) crossing into Canada from the US overland, again I get a 90 day visa and I'm guessing that my plane ticket would definately satisfy border control. So there shouldn't be any issue for this section of the trip. Am I right?

Any help appreciated. Thanks.