Take two: I could manage to run a marathon
An dàrna turas: Dhèanainn a' chùis air maraton a ruith
Let's have a look at this conversation again.

An canadh tu gu bheil thu lùthmhor, Fhionnlaigh?
Would you say that you are athletic, Finlay?

Chanadh. Dhèanainn a' chùis air maraton a ruith. Tha mi a' dol ga dhèanamh am–bliadhna.
I would [say]. I could manage to run a marathon. I'm going to do it this year.

An deidheadh agadsa air maraton a ruith, a Mhàiri?
Would you manage to run a marathon, Màiri?

Cha deidheadh agam air maraton a ruith. 'S urrainn dhomh 5k [còig cileameatair] a dhèanamh, ge–tà.
I wouldn't manage to run a marathon. I am able to do a 5k, though.

Tha sin math! Am b' urrainn dhut nas fhaide na sin a dhèanamh, le cleachdadh?
That's good! Would you be able to do further than that, with practice?

B' urrainn, tha mi cinnteach. Aig an toiseach cha robh e comasach dhomh coiseachd gu deireadh na sràide!
I could, I'm sure. At the beginning, it wasn't possible for me to walk to the end of the street!

Thèid agam air cuairt a dhèanamh dhen Eilean Sgitheanach ann an dà latha air a' bhaidhsagal.
I can do a circuit of [the Isle of] Skye in two days on the bike.

Iongantach!
Amazing!

B' urrainn dhomh snàmh sa mhuir nuair a bha mi nam phàiste. Dheidheadh agam air mìle a shnàmh gu taobh thall an locha—leam fhèin. Agus dheidheadh agam air tilleadh cuideachd!
I could swim in the sea when I was a kid. I would manage to swim a mile to the other side of the loch—by myself. And I would manage to get back [return] as well!

An tèid agad air sin a dhèanamh an–diugh, saoil?
Can you [will you] manage to do that today, do you think?

Thèid agam air snàmh san amar na làithean seo! Ach le beagan cleachdaidh, cò aige a tha fios? Math dh'fhaodte gun tèid.
I can manage to swim in the bath these days! But with a bit of practice, who knows? Perhaps I can [manage].

Air a' chroit, dhèanainn a' chùis air còig mìle gach latha—an tòir air coin is caoraich!
On the croft, I’d manage to do five miles every day—chasing dogs and sheep!